This application claims a foreign priority based on German patent application 199 25 049.9, filed Jun. 1, 1999, and the disclosure in that application is expressly incorporated by reference herein.
The present invention relates to plastic cover plates for motor vehicle headlights or other motor vehicle lights.
The manufacture of motor vehicle headlights and motor vehicle lights on the basis of thermoplastic raw materials has been known for a long time. In recent years, plastic cover plates made of polycarbonate have found wide acceptance. However, in actual vehicular usage, specking has been observed on the inside of the plastic plates, that is, on the surface facing the light source. This involves essentially dust deposits, which cause a disruptive impairment within systems that are not completely closed.
In a ventilated system of a motor vehicle headlight or light, the flecks are caused especially by a varying vaporization of additives in the thermoplastic materials. In operation, temperatures ranging from 120 to 125xc2x0 C. can occur in the plastic plate. At sites which are especially hot, greater vaporization of the additives takes place than occurs in the colder zones. Here, possibly, traces of the additives settle, so that the inner surface no longer has the same static properties throughout. In this way, variable dust deposition occurs.
The purpose of the invention presented here consists in preventing specking, which is obviously disruptive, especially in non-profile, clear cover plates of motor vehicle headlights. Of course, efforts have been made among the manufacturers, who manufacture the raw materials for the production of plastic plates, to develop plastics, which prevent specking through corresponding material components. However, these material combinations, as they are described for example in DE 197 13 508 A1, are not completely satisfactory.
For transparent thermoplastics, internal antistatic agents, which are added to the raw material, cannot be used without impairing properties of the cover plate material. Externally applied antistatic agents are effective only temporarily, and are affected the air humidity present in the light assembly.
The aforementioned purposexe2x80x94the prevention of speckingxe2x80x94is achieved in a first embodiment form through a plastic plate of a motor vehicle headlight or motor vehicle light, which is characterized in that the plate has, on the surface facing towards the light, a transparent coating to prevent specks from forming through a non-homogeneous deposition of dust.
Using the invention presented here, a non-visible, completely transparent coating is thus made available for the inner surface of plastic plates of motor vehicle headlights or motor vehicle lights, which in the installed condition are no longer accessible for cleaning and because of electrostatic charging, tend to collect dust particles in the form of specks that are thus obvious decoratively.
With the aid of the invention presented here, it was discovered that this dust deposition can then be prevented especially when the coating is made of SiO.SiO2(Si2O3). This material, whose fundamental application is known for the manufacture of a reflection reducing layer on lenses, is used according to the present invention primarily in the sector of the motor vehicle industry. Surprisingly, it has been discovered that very thin layers of this material, for example, 5 to 100 nm, in particular, 25 to 50 nm, are suitable for preventing flecking to the greatest extent possible.
The application of the coating occurs according to processes described in the state of the art, as are disclosed, for example, in the patent DE 41 28 547 A1, the entire contents of which are hereby expressly incorporated by reference.